Mozilla will drop support for Apple's OS X 10.5, or Leopard, after it ships Firefox 16 in October, according to company developers.

"We are not planning to support Mac OS X 10.5 with Firefox 17," said Josh Aas, who works on the Firefox platform group, in a message last month on Bugzilla. "The builds will fail to run on anything less than Mac OS X 10.6."

OS X 10.6 is Snow Leopard, the 2009 follow-up to Leopard, which shipped in October 2007.

By Mozilla's release calendar, Firefox 16 is to debut Oct. 9. Firefox 17, the first that will not to pushed to Leopard users, is slated for a Nov. 20 launch.

According to Mozilla, Leopard's importance is diminishing. "Mac OS X 10.5 users have been declining by 1% per month, as a share of our total Mac OS X users," said Aas. "This, combined with the impact of the release of Mac OS X 10.8 [Mountain Lion], means that Mac OS X 10.5 users will likely make up around 10% of Mac OS X users when Firefox 17 ships."

As of June 21, 17% of Firefox 13's Mac users were running Leopard, with larger shares on Snow Leopard (35%) and Lion (48%), Aas said. Only 4.6% of all Firefox 13 users were running it on a Mac.

Like Google, another reason Mozilla cited for dumping Leopard was that Apple has also ended support.

The last time Apple patched bugs in Leopard was November 2011, and its most recent security update, in May 2012, disabled older copies of Flash Player to stymie Flashback rather than fix specific security flaws. Nor has Apple maintained Safari on OS X 10.5. The final update was issued over a year ago.

And finally, said Aas, Mozilla can handle only so much.

"Apple releases new versions of its operating systems relatively quickly, and each new version contains significant changes that we must adapt to," Aas said. "This requires resources, and with limited resources this sometimes means we have to make tough decisions about where to invest."

One project that Mozilla has been pushing is a browser compatible with Windows 8, Microsoft's new operating system.

In a blog post last week, Brian Bondy, a Firefox platform engineer who has been working on a "Metro" version for Windows 8, said that the browser has "progressed steadily," adding that Mozilla's goal is to ship a preview by the end of next month.

"This preview will include primary browser UI for navigation and tabs, and will be delivered as a combined classic + Metro browser," said Bondy. "I believe there is a Q4 goal for a beta release as well."

Windows 8, which is available now to some Microsoft customers, hits retail on Oct. 26.

Mozilla was the first Microsoft rival to announce it would create a combination desktop and Metro-style browser to compete with Internet Explorer 10 (IE10) on Windows 8.